The present invention relates to a vacuum circuit breaker and, more particularly, to a vacuum circuit breaker having electrodes in which a contact portion impregnated with an alloy is joined to a conductive support member.
In a vacuum circuit breaker it is desirable to have a small chopping current value and a low surge voltage caused in an electric load by breaking the electric current. In order to obtain the desired operating characteristics, improvements mainly in the materials of the electrodes have been proposed the prior art to propose a variety of electrode materials. More particularly, for example, Japanese Patent Laid-Open No. 5928/1983, proposes an impregnating alloy of Co-Ag-Te or Se, whereby electrodes made of the disclosed alloy have a low surging property (in which the chopping current value is so low that the surge voltage to a load device is low) and has a withstanding excellent voltage characteristic and current breaking capacity. That alloy is prepared by lightly sintering Co powder in advance in a non-oxidizing atmosphere and by vacuum-impregnating the sintered porous product with an alloy of Ag-Te or Ag-Se. An electrode has a high conducting capacity if it is made exclusively of the material thus prepared, because this material has a higher electrical resistance than that of an electrode material composed mainly of copper or silver. Therefore, the material is so joined to a conductive member to form an electrode that it is used only as a contact portion. This joining is performed by a soldering method. A variety of soldering methods have been investigated to determine the manner by which an impregnating alloy having a small concentration of Te or Se can be joined by a general Ag soldering method (i.e., BAg-8 according to the Japanese Industrial Standards). It has been found that the impregnating alloy can hardly be soldered if the concentration of Te or Se exceeds 10 wt. %. This is thought to come from the fact that Te or Se in the impregnating alloy enters the joined layer to make the layer fragile in its entirety. Even if the concentration of Te or Se is lower than the above-specified weight percentage, moreover, there is a tendency that the joining strength becomes weaker than the usual soldering strength. Still moreover, the soldering material has a tendency to diffuse and penetrate into the impregnating alloy thereby raising a problem that the initial composition cannot be maintained to shift the electrode performance. This phenomenon is also caused in case a contact point, in which a porous sintered product of other than Co (e.g., Fe, Ni or Cr) is impregnated with one of alloys of Ag-Pb, Ag-Bi and Ag-Cd. Thus, the contact material prepared by impregnating a sintered product of a refractory metal with the Ag alloy has a problem in the solderability despite it exhibits excellent characteristics as the electrodes of a low-surge vacuum circuit breaker.
An object of the present invention resides in providing a vacuum circuit breaker including electrodes which has a contact portion of a sintered porous body impregnated with an alloy joined firmly to a conductive support portion so that it can withstand a strong peeling force.
According to the present invention, a vacuum circuit breaker which is equipped with a pair of electrodes arranged in a vacuum container to face each other, with each of the electrodes being constructed of a support electrode, an auxiliary support electrode joined to the support electrode, and an electrical contact portion made of a sintered refractory, porous sintered body on the auxiliary support electrode and a conductive metal impregnating said sintered body. The auxiliary support electrode has a protrusion which is shaped to laterally extend at an end thereof and provided on the electrical contact portion side of the auxiliary support electrode.
Preferably, the auxiliary support electrode is joined to the support electrode by the soldering method and is operative to provide a barrier in case of the soldering.
The auxiliary support electrode serves as a barrier against penetration of the impregnating conductive metal into a joining face when soldering is effected, and has the protrusion joined to the porous refractory cobalt body of the electrical contact portion, thereby preventing separation between the auxiliary support electrode and the porous refractory cobalt body at the sintered face and between the electrical contact portion and the support electrode at the joined surface.